| title | keywords | |||||||
|---|---|---|---|---|---|---|---|---|
Bash scripting |
|
{: .-three-column}
#!/usr/bin/env bash
NAME="John"
echo "Hello $NAME!"NAME="John"
echo $NAME
echo "$NAME"
echo "${NAME}!"NAME="John"
echo "Hi $NAME" #=> Hi John
echo 'Hi $NAME' #=> Hi $NAMEecho "I'm in $(pwd)"
echo "I'm in `pwd`"
# Samegit commit && git push
git commit || echo "Commit failed"{: id='functions-example'}
get_name() {
echo "John"
}
echo "You are $(get_name)"See: Functions
{: id='conditionals-example'}
if [[ -z "$string" ]]; then
echo "String is empty"
elif [[ -n "$string" ]]; then
echo "String is not empty"
fiSee: Conditionals
set -euo pipefail
IFS=$'\n\t'See: Unofficial bash strict mode
echo {A,B}.js| {A,B} | Same as A B |
| {A,B}.js | Same as A.js B.js |
| {1..5} | Same as 1 2 3 4 5 |
See: Brace expansion
{: .-three-column}
name="John"
echo ${name}
echo ${name/J/j} #=> "john" (substitution)
echo ${name:0:2} #=> "Jo" (slicing)
echo ${name::2} #=> "Jo" (slicing)
echo ${name::-1} #=> "Joh" (slicing)
echo ${name:(-1)} #=> "n" (slicing from right)
echo ${name:(-2):1} #=> "h" (slicing from right)
echo ${food:-Cake} #=> $food or "Cake"length=2
echo ${name:0:length} #=> "Jo"See: Parameter expansion
STR="/path/to/foo.cpp"
echo ${STR%.cpp} # /path/to/foo
echo ${STR%.cpp}.o # /path/to/foo.o
echo ${STR##*.} # cpp (extension)
echo ${STR##*/} # foo.cpp (basepath)
echo ${STR#*/} # path/to/foo.cpp
echo ${STR##*/} # foo.cpp
echo ${STR/foo/bar} # /path/to/bar.cppSTR="Hello world"
echo ${STR:6:5} # "world"
echo ${STR:-5:5} # "world"SRC="/path/to/foo.cpp"
BASE=${SRC##*/} #=> "foo.cpp" (basepath)
DIR=${SRC%$BASE} #=> "/path/to/" (dirpath)| Code | Description |
|---|---|
${FOO%suffix} |
Remove suffix |
${FOO#prefix} |
Remove prefix |
| --- | --- |
${FOO%%suffix} |
Remove long suffix |
${FOO##prefix} |
Remove long prefix |
| --- | --- |
${FOO/from/to} |
Replace first match |
${FOO//from/to} |
Replace all |
| --- | --- |
${FOO/%from/to} |
Replace suffix |
${FOO/#from/to} |
Replace prefix |
# Single line comment: '
This is a
multi line
comment
'| ${FOO:0:3} | Substring (position, length) |
| ${FOO:-3:3} | Substring from the right |
| ${#FOO} | Length of $FOO |
STR="HELLO WORLD!"
echo ${STR,} #=> "hELLO WORLD!" (lowercase 1st letter)
echo ${STR,,} #=> "hello world!" (all lowercase)
STR="hello world!"
echo ${STR^} #=> "Hello world!" (uppercase 1st letter)
echo ${STR^^} #=> "HELLO WORLD!" (all uppercase)| ${FOO:-val} | $FOO, or val if not set |
| ${FOO:=val} | Set $FOO to val if not set |
| ${FOO:+val} | val if $FOO is set |
| ${FOO:?message} | Show error message and exit if $FOO is not set |
The : is optional (eg, ${FOO=word} works)
{: .-three-column}
for i in /etc/rc.*; do
echo $i
donefor ((i = 0 ; i < 100 ; i++)); do
echo $i
donefor i in {1..5}; do
echo "Welcome $i"
donefor i in {5..50..5}; do
echo "Welcome $i"
donecat file.txt | while read line; do
echo $line
donewhile true; do
···
done{: .-three-column}
myfunc() {
echo "hello $1"
}# Same as above (alternate syntax)
function myfunc() {
echo "hello $1"
}myfunc "John"myfunc() {
local myresult='some value'
echo $myresult
}result="$(myfunc)"myfunc() {
return 1
}if myfunc; then
echo "success"
else
echo "failure"
fi| Expression | Description |
|---|---|
$# |
Number of arguments |
$* |
All arguments |
$@ |
All arguments, starting from first |
$1 |
First argument |
$_ |
Last argument of the previous command |
See Special parameters.
{: .-three-column}
Note that [[ is actually a command/program that returns either 0 (true) or 1 (false). Any program that obeys the same logic (like all base utils, such as grep(1) or ping(1)) can be used as condition, see examples.
| Condition | Description |
|---|---|
[[ -z STRING ]] |
Empty string |
[[ -n STRING ]] |
Not empty string |
[[ STRING == STRING ]] |
Equal |
[[ STRING != STRING ]] |
Not Equal |
| --- | --- |
[[ NUM -eq NUM ]] |
Equal |
[[ NUM -ne NUM ]] |
Not equal |
[[ NUM -lt NUM ]] |
Less than |
[[ NUM -le NUM ]] |
Less than or equal |
[[ NUM -gt NUM ]] |
Greater than |
[[ NUM -ge NUM ]] |
Greater than or equal |
| --- | --- |
[[ STRING =~ STRING ]] |
Regexp |
| --- | --- |
(( NUM < NUM )) |
Numeric conditions |
| Condition | Description |
|---|---|
[[ -o noclobber ]] |
If OPTIONNAME is enabled |
| --- | --- |
[[ ! EXPR ]] |
Not |
[[ X ]] && [[ Y ]] |
And |
| `[[ X ]] |
| Condition | Description |
|---|---|
[[ -e FILE ]] |
Exists |
[[ -r FILE ]] |
Readable |
[[ -h FILE ]] |
Symlink |
[[ -d FILE ]] |
Directory |
[[ -w FILE ]] |
Writable |
[[ -s FILE ]] |
Size is > 0 bytes |
[[ -f FILE ]] |
File |
[[ -x FILE ]] |
Executable |
| --- | --- |
[[ FILE1 -nt FILE2 ]] |
1 is more recent than 2 |
[[ FILE1 -ot FILE2 ]] |
2 is more recent than 1 |
[[ FILE1 -ef FILE2 ]] |
Same files |
# String
if [[ -z "$string" ]]; then
echo "String is empty"
elif [[ -n "$string" ]]; then
echo "String is not empty"
fi# Combinations
if [[ X ]] && [[ Y ]]; then
...
fi# Equal
if [[ "$A" == "$B" ]]# Regex
if [[ "A" =~ . ]]if (( $a < $b )); then
echo "$a is smaller than $b"
fiif [[ -e "file.txt" ]]; then
echo "file exists"
fiFruits=('Apple' 'Banana' 'Orange')Fruits[0]="Apple"
Fruits[1]="Banana"
Fruits[2]="Orange"echo ${Fruits[0]} # Element #0
echo ${Fruits[@]} # All elements, space-separated
echo ${#Fruits[@]} # Number of elements
echo ${#Fruits} # String length of the 1st element
echo ${#Fruits[3]} # String length of the Nth element
echo ${Fruits[@]:3:2} # Range (from position 3, length 2)Fruits=("${Fruits[@]}" "Watermelon") # Push
Fruits+=('Watermelon') # Also Push
Fruits=( ${Fruits[@]/Ap*/} ) # Remove by regex match
unset Fruits[2] # Remove one item
Fruits=("${Fruits[@]}") # Duplicate
Fruits=("${Fruits[@]}" "${Veggies[@]}") # Concatenate
lines=(`cat "logfile"`) # Read from filefor i in "${arrayName[@]}"; do
echo $i
done{: .-three-column}
declare -A soundssounds[dog]="bark"
sounds[cow]="moo"
sounds[bird]="tweet"
sounds[wolf]="howl"Declares sound as a Dictionary object (aka associative array).
echo ${sounds[dog]} # Dog's sound
echo ${sounds[@]} # All values
echo ${!sounds[@]} # All keys
echo ${#sounds[@]} # Number of elements
unset sounds[dog] # Delete dogfor val in "${sounds[@]}"; do
echo $val
donefor key in "${!sounds[@]}"; do
echo $key
doneset -o noclobber # Avoid overlay files (echo "hi" > foo)
set -o errexit # Used to exit upon error, avoiding cascading errors
set -o pipefail # Unveils hidden failures
set -o nounset # Exposes unset variablesshopt -s nullglob # Non-matching globs are removed ('*.foo' => '')
shopt -s failglob # Non-matching globs throw errors
shopt -s nocaseglob # Case insensitive globs
shopt -s dotglob # Wildcards match dotfiles ("*.sh" => ".foo.sh")
shopt -s globstar # Allow ** for recursive matches ('lib/**/*.rb' => 'lib/a/b/c.rb')Set GLOBIGNORE as a colon-separated list of patterns to be removed from glob
matches.
| history | Show history |
| shopt -s histverify | Don't execute expanded result immediately |
| !$ | Expand last parameter of most recent command |
| !* | Expand all parameters of most recent command |
| !-n | Expand nth most recent command |
| !n | Expand nth command in history |
| !<command> | Expand most recent invocation of command <command> |
| !! | Execute last command again |
| !!:s/<FROM>/<TO>/ | Replace first occurrence of <FROM> to <TO> in most recent command |
| !!:gs/<FROM>/<TO>/ | Replace all occurrences of <FROM> to <TO> in most recent command |
| !$:t | Expand only basename from last parameter of most recent command |
| !$:h | Expand only directory from last parameter of most recent command |
!! and !$ can be replaced with any valid expansion.
| !!:n | Expand only nth token from most recent command (command is 0; first argument is 1) |
| !^ | Expand first argument from most recent command |
| !$ | Expand last token from most recent command |
| !!:n-m | Expand range of tokens from most recent command |
| !!:n-$ | Expand nth token to last from most recent command |
!! can be replaced with any valid expansion i.e. !cat, !-2, !42, etc.
$((a + 200)) # Add 200 to $a$((RANDOM%=200)) # Random number 0..200(cd somedir; echo "I'm now in $PWD")
pwd # still in first directorypython hello.py > output.txt # stdout to (file)
python hello.py >> output.txt # stdout to (file), append
python hello.py 2> error.log # stderr to (file)
python hello.py 2>&1 # stderr to stdout
python hello.py 2>/dev/null # stderr to (null)
python hello.py &>/dev/null # stdout and stderr to (null)python hello.py < foo.txt # feed foo.txt to stdin for pythoncommand -V cd
#=> "cd is a function/alias/whatever"trap 'echo Error at about $LINENO' ERRor
traperr() {
echo "ERROR: ${BASH_SOURCE[1]} at about ${BASH_LINENO[0]}"
}
set -o errtrace
trap traperr ERRcase "$1" in
start | up)
vagrant up
;;
*)
echo "Usage: $0 {start|stop|ssh}"
;;
esacsource "${0%/*}/../share/foo.sh"printf "Hello %s, I'm %s" Sven Olga
#=> "Hello Sven, I'm Olga
printf "1 + 1 = %d" 2
#=> "1 + 1 = 2"
printf "This is how you print a float: %f" 2
#=> "This is how you print a float: 2.000000"DIR="${0%/*}"while [[ "$1" =~ ^- && ! "$1" == "--" ]]; do case $1 in
-V | --version )
echo $version
exit
;;
-s | --string )
shift; string=$1
;;
-f | --flag )
flag=1
;;
esac; shift; done
if [[ "$1" == '--' ]]; then shift; ficat <<END
hello world
ENDecho -n "Proceed? [y/n]: "
read ans
echo $ansread -n 1 ans # Just one character| $? | Exit status of last task |
| $! | PID of last background task |
| $$ | PID of shell |
| $0 | Filename of the shell script |
See Special parameters.
pwd # /home/user/foo
cd bar/
pwd # /home/user/foo/bar
cd -
pwd # /home/user/fooif ping -c 1 google.com; then
echo "It appears you have a working internet connection"
fiif grep -q 'foo' ~/.bash_history; then
echo "You appear to have typed 'foo' in the past"
fi{: .-one-column}
- Bash-hackers wiki (bash-hackers.org)
- Shell vars (bash-hackers.org)
- Learn bash in y minutes (learnxinyminutes.com)
- Bash Guide (mywiki.wooledge.org)
- ShellCheck (shellcheck.net)